Paroxysmal nocturnal hemoglobinuria (PNH) is a blood disorder, which is caused by the clonal expansion of a hematopoietic progenitor cell that carries a somatic mutation in the X-linked PIGA gene. It presented classically with hemoglobinuria due to intravascular hemolysis, thrombotic complications, and pancytopenia. The PIGA gene encodes a protein subunit of a glycosyltransferase essential in the synthesis of glycosyl phosphatidylinositol (GPI) anchor molecules. Patients with PNH therefore have a proportion of blood cells deficient in all GPI-linked surface molecules. PNH is frequently found in patients with aplastic anemia (AA) and in patients with myelodysplasia (MDS). Although not a neoplastic disease on its own, patients with PNH have an increased risk of developing acute myeloid leukemia (AML). Promoted by the clinical association of PNH with AA, MDS, and AML, we raised the hypothesis that a PIGA gene mutation alone does not cause clonal expansion or leukemic transformation. But due to their inability to like certain proteins to the cell surface through a GPI-anchor PNH cells escape immuno surveillance and cell death that causes bone marrow aplasia in AA and controls neoplastic cell growth in early leukemogenesis. In the proposed research we will use a mouse model that closely mimics the human disease and investigate the association of PNH with MDS and AML. We will obtain mice with blood cells lacking GPI-linked proteins by disrupting the murine Piga gene in early hematopoietic progenitor cells in the bone marrow using the Cre-loxP system. By this approach we will generate two types of mice, one with all blood cells deficient in GPI-linked proteins whereas the other will have both PIGA (+) and PIGA(-) circulating blood cells. We will then compare PIGA(+) and PIGA(-) hematopoiesis in these mice in vitro and in vivo under a variety of circumstances, including the administration of stimuli that trigger cell death along with agents known to cause leukemia transformation. Competition between cells expressing wild type Piga and those expressing the recombined Piga allele will enable us to uncover even subtle differences in cell death and proliferation in any stages of hematopoietic differentiation. These experiments will demonstrate whether PIGA(-) blood cells are more resistant to specific stimuli that activate apoptotic cell death and whether mice with PIGA(-) blood cells develop leukemia earlier and more frequent compared to mice with phenotypically normal blood cells. In this way we hope to identify the factors that differentially influence growth and death of PNH and normal hematopoietic progenitor cells and to elucidate mechanisms that may lead to leukemia transformation in patients with PNH. The availability of a mouse model for PNH will provide us with a powerful tool to test new therapeutic agents for the treatment of PNH, PNH/MDS, PNH,AML and possibly other clonal blood disorders.